Secondary ion mass spectrometry measurements of volcanic tuffs containing radionuclides from underground nuclear tests

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The in situ distribution of anthropogenic radionuclides in volcanic tuffs was measured using secondary ion mass spectrometry (SIMS). Samples were obtained from boreholes drilled in close proximity to expended underground nuclear test cavities at the Nevada Test Site, USA. SIMS measurements revealed the presence of22Na,137Cs, and enriched235U/238U ratios. The radionuclides are distributed heterogeneously and occur at concentrations ranging from 1 to <0.1 ppm. Two processes govern the distribution of radionuclides outside the nuclear test cavity region: the prompt injection of radionuclides along fractures at the time of the nuclear detonation, and the post-test sorption of radionuclides from groundwater. A zone of prompt injection 37 m above the static water table is characterized by the correlated presence of all three radionuclides - in one case, all within a single (60 μm) point of analysis.137Cs is the only radionuclide detected in rocks from the saturated zone. Since these volcanic tuffs are highly zeolitized, containing up to 60 wt % clinoptilolite, it is likely that zeolite minerals control137Cs sorption from groundwater. However, the heterogeneous distribution of radionuclides within the zeolitized samples indicates that micro-fractures in the rock are also important in determining where the radionuclides are deposited.